[A]s New Yorkers, we cannot and will not abandon our waterfront. It’s one of our greatest assets. We must protect it, not retreat from it.

Of course, who could ever imagine a politician standing on a coastline proclaiming, “We will retreat!”

Photo

A flooded tunnel entrance in Lower Manhattan after the storm surge from Hurricane Sandy hit in 2012.Credit Damon Winter/The New York Times

But somehow, that’s what has to be done. Finding a way to have a realistic discussion of where to hold firm and where to pull back, where to gird and where to let nature dominate, has to happen to limit costs and other regrets in thousands of coastal cities and smaller communities around the world.

Population growth, urbanization and rising sea levels are building enormous vulnerability to flooding in Malé, a tiny island that is the capital of the Indian Ocean nation of Maldives. (Click for larger version and National Geographic article.)Credit George Steinmetz

“The problem is we’re still building the city of the past,” says Jacob. “The people of the 1880s couldn’t build a city for the year 2000—of course not. And we cannot build a year-2100 city now. But we should not build a city now that we know will not function in 2100. There are opportunities to renew our infrastructure. It’s not all bad news. We just have to grasp those opportunities.”

Clarifying the pace of coastal retreats is incredibly important in judging how much, and how, to invest at any given time to limit losses over the long run. Unfortunately, decades of scientific research have not substantially determined what’s likely in the near term (by 2100) even as the long-term picture of inundation remains robust. Revisit my video discussions with ocean analyst Josh Willis of NASA to learn more.

Most forecasting is easier and more reliable in the short run than over the long haul. Think of weather prediction. (And history is full of failed long-term forecasts of everything from oil prices to human population trends.)

But for scientists studying the fate of the vast ice sheets of Greenland and West Antarctica, the situation seems reversed. Their views of sea trends through this century still vary widely, while they agree, almost to a person, that centuries of eroding ice and rising seas are nearly a sure thing in a warming world…

My emails and calls to more than a dozen experienced ice scientists produced about a 50/50 split on whether Greenland or Antarctica was the biggest short-term risk.

But there was little disagreement that playing what amounts to two games of high-stakes poker at the same time by driving up greenhouse-gas concentrations is a bad idea, particularly as ever more people concentrate on coastlines in both rich and poor countries.

So where does this leave you if you live near a shore or run a coastal port, airport or city? Is the Bloomberg plan a good one or a stopgap that fits political imperatives of the moment while building bigger risks in the long haul?

Flood exposure is increasing in coastal cities owing to growing populations and assets, the changing climate, and subsidence Here we provide a quantification of present and future flood losses in the 136 largest coastal cities. Using a new database of urban protection and different assumptions on adaptation, we account for existing and future flood defenses. Average global flood losses in 2005 are estimated to be approximately $6 billion per year (U.S. dollars), increasing to $52 billion by 2050 with projected socio-economic change alone.

With climate change and subsidence, present protection will need to be upgraded to avoid unacceptable losses. Even if adaptation investments maintain flood probability, subsidence and sea-level rise will increase mean annual losses by between 15 and 22 percent in 2050 on top of the increase due to socioeconomic changes. To maintain flood risk, adaptation will need to reduce flood probabilities below present values. In this case, the magnitude of losses when floods do occur would increase, often by more than 50 percent, making it critical to also prepare for larger disasters than we experience today. The analysis identifies the cities that seem most vulnerable to these trends, that is, where the largest increase in losses can be expected.

Hallegatte sent three findings translated from paper jargon to language that even a mayor could understand:

– For the first time, this study takes into account existing coastal defenses. And we find that because of these defenses — and the way they have been designed for the current environmental conditions — the cities are very vulnerable to even moderate changes in sea level. Cities that are very well protected today are particularly vulnerable to such a change. Compared with previous studies that found that losses increase regularly with sea-level rise, we find a much sharper increase with sea level because of this effect. In policy terms, what it shows is that major investments in coastal protections will be needed in the next decades, at a high cost ($50 billion per year for the 136 cities). This is probably easy for rich cities, but it is more challenging for poor countries.

– The cities where the risk will increase most are not the cities where the risk is particularly high today (such as around the Mediterranean basin). So, cities where flood risk is not a priority today will have to take this problem seriously. And it is a challenge: one cannot see an increase in risk; what we see is the disaster when it is too late. The challenge for these cities is to do something about the increase in risk before the disaster hit. We know that this is politically difficult.

– Adaptation can decrease the probability of a disaster occurring, but all the cities will become more vulnerable to a failure in defenses, or to an exceptional event that exceeds the defense design. It means that floods – when they occur – will be larger in the future. It calls for better crisis management and contingency planning (including early warning systems and evacuation), reconstruction planning, and international collaboration, especially when poor countries are affected (or when small countries are hit, since for them it can be the entire national economy that is stalled by a flood, making them unable to manage the recovery and reconstruction).

As a side result, the U.S. appears particularly vulnerable, with defense standards much lower than countries with similar income (and even lower than many developing countries).

[A]n important limit of adaptation is that it can reduce the probability of floods (by raising defenses), but it also increases the losses when a flood does occur (that is, when the defenses fail or are exceeded by an exceptional event). Basically, with adaptation we’re moving to a world that is better protected, and therefore more dependent on these protections, and more vulnerability to their failing.

Getting comfortable with the reality of no new normal coastline is clearly an imperative in the Anthropocene, this era (long or short; it’s up to us) in which Earth is increasingly a human-shaped system.

Update, 3:53 p.m. | I just recalled that my old friend Mike Lemonick wrote an excellent piece in 2012 for Climate Central focused on the response of coastal communities in Florida to sea-level realities. Here’s the opener and a link to the rest:

It’s not unusual for Keith London to run into people who doubt that global warming is really such a big deal. “I tell them, ‘the ocean is rising,’ ” he said. “They say, ‘so?’ It drives you crazy.”

London is no scientist; he’s a city commissioner in Hallandale Beach, Fla., a municipality of about 37,000 that sits on the Atlantic coast between Fort Lauderdale and Miami. But he talks to scientists and engineers all the time as part of his job, and the story they tell him isn’t pretty. “The average elevation in Florida is 6 feet,” London said. “Some places are as little as 3 feet above sea level. And sea level is going to rise as all that ice in the Arctic melts.” [Read the rest.]

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By 2050 or so, the human population is expected to pass nine billion. Those billions will be seeking food, water and other resources on a planet where humans are already shaping climate and the web of life. Dot Earth was created by Andrew Revkin in October 2007 -- in part with support from a John Simon Guggenheim Fellowship -- to explore ways to balance human needs and the planet's limits.